The present invention generally relates to semiconductor structures and, more particularly, to semiconductor structures including metal migration barrier layers preventing metal migration from a contact layer into an active layer of a semiconductor structure and method of forming the same.
While manufacturing semiconductor devices, such as III–V semiconductor based multifunction solar cells, electrical contacts are formed on individual semiconductor devices and connected together to perform the desired circuit functions. The electrical contact formation and connection process involves metal layers and is generally called “metallization”. Metal contact layers containing multiple thin layers with alternating composition of, for example Ti/Au/Ag, are conventionally used for providing electrical contact to semiconductor devices.
FIG. 1 provides a schematic cross sectional view of a typical prior art semiconductor structure 10. The semiconductor layers may be deposited during a process typically referred to as semiconductor wafer growth process. In this wafer growth process, semiconductor active layers 11 are generally deposited onto a semiconductor substrate 12. A semiconductor contact layer 13 may be then deposited onto the semiconductor active layers 11. Typically, the semiconductor contact layer 13 is the last layer to be deposited during the wafer growth process. After completion of the growth process, a metal contact 14 is generally deposited in a separate process typically referred to as device fabrication process.
Metal films used for electrical contacts on semiconductor devices may migrate into the semiconductor active region under certain environmental conditions, such as long time exposure to high temperatures, causing product reliability concerns. FIG. 2 shows a schematic cross sectional view of a typical prior art semiconductor structure 10 wherein metal protrusion 15 from the metal contact 14 into the semiconductor active layers 11 has occurred. The metal protrusion 15 may cause the semiconductor structure 10 to fail. Consequently, a semiconductor structure that blocks or prevents metal migration from the metal contact into the semiconductor active layer is highly desirable and is important for the operation life, and high temperature reliability of such semiconductor structure 10, for example, a solar cell.
The most common method to mitigate the existing problem is to use a metal barrier layer as part of the entire metal contact structure, such that the barrier metal is placed underneath the main metal layer 14. In prior art, platinum (Pt), palladium (Pd), and similar elements or compounds that have high temperature stability are commonly used as metal barrier layers. The main purpose of the metal barrier layer is to prevent diffusion of conductive material from the metal contact 14, such as Au or Ag, for example, into the semiconductor active layers 11.
However, this prior art approach may still be susceptible to metal diffusion under certain environmental conditions, such as exposure of the semiconductor structure to high temperatures for prolonged periods of time. Since the semiconductor contact layer 13 generally is composed of a single homogeneous structure, the semiconductor contact layer 13 may not provide any obstacles to the metal protrusion 15 once the metal diffusion process has started. Therefore, once metal migration from the metal contact 14 into the semiconductor contact layer 13 has started, it is likely that the metal will migrate through the entire semiconductor contact layer 13 and reach the active semiconductor region. Consequently, it will be only a matter of time until the metal reaches the semiconductor active layers 11 and causes the structure to fail. Therefore, it is necessary to find appropriate materials that can be incorporated within the contact structure of a semiconductor structure and that enable suppression of the metal migration into the active region of the structure even during long time exposure to high temperatures, as found for example during space and terrestrial applications.
Prior art multijunction solar cells 11 (as shown in FIG. 3) provide power to many satellites and other spacecraft. FIG. 3 shows a perspective view of a cell-interconnect-coverglass assembly 30 of a typical prior art multijunction solar cell 31. A plurality of interconnects 32 may be provided at one edge of the solar cell 31. The interconnects 32 may be welded on top of the metal contact 14, as shown in FIGS. 1 and 2. A coverglass 33 may be installed to protect the solar cell 31 and the interconnects 32 from radiation in space. Presently, metal protrusion 15 from the metal contact 14 through the semiconductor contact layer 13 and into the semiconductor active layers 11, as illustrated in FIG. 2, may be observed more readily in the areas where the interconnects 32 are connected with the solar cell 31. For space applications, such as the operation of III–V based multijunction solar cells mounted on spacecraft, the device operation life under extreme environmental conditions, such as exposure to relatively high temperatures for relatively long times, is of very high importance. Therefore, a prolonged life, a higher performance, an improved high temperature reliability, and stability of the solar cells 31 would result in a prolonged operation of the spacecraft. Furthermore, solar cells 31 with a semiconductor contact structure that blocks or prevents metal protrusion 15 would provide a more stable and improved total power output over the life of a spacecraft due to a lower degradation rate.
There has, therefore, arisen a need to provide a metal migration semiconductor barrier layer that is able to suppress metal migration from the metal contact of a semiconductor device into the semiconductor active region under extreme environmental conditions, such as exposure to high temperatures for long times, as found, for example, for applications in space, as well as on earth, under concentrated sunlight. There has further arisen a need to provide an appropriate material for a metal migration semiconductor barrier layer that is able to block the movement of metal within the semiconductor contact layer and to keep the metal away from the active region of the semiconductor device under extreme environmental conditions, such as found in space. There has also arisen a need to provide an improved solar cell for providing power to a spacecraft, such as a satellite, that will prolong the operation and improve the performance of the spacecraft.
As can be seen, there is a need for a semiconductor structure with a semiconductor contact structure providing improved high temperature reliability. There is a further need for providing a semiconductor structure designed to keep any metal from entering the semiconductor active region such that, consequently, the operation life of the semiconductor structure will be extended. Also, there is a need for preventing metal protrusion from the metal contact into the semiconductor active layers during exposure of the semiconductor structure to high temperatures for long times, improving the performance and stability of the structure. Furthermore, there is a need for suppression of metal migration into the semiconductor active layers under extreme environmental conditions, such as exposure to high temperatures for long times as found, for example, during space applications. Moreover, there is a need for a method for forming a semiconductor structure for improving the high temperature reliability and the performance time of the semiconductor structure.